Alot of what Marth does, Lucina can do about the same, so try using running shields to break off the projectiles and keep rushing forward. You do this by holding down the direction you're running towards, then press and hold the shield button to slide to a stop with the shield up. This is very useful, especially for the next part.
When you know they keep rolling, run after them and bait them to keep rolling. There's a brief moment after they roll that they're open, so go for the running shied > grab. If you wiff, you're still safe since you're using a standing still grab instead of a running grab.
Be weary of using the ftilt though, as though it does cover a good space, it does have frames that you are left out in the open. Use it to tech read opponents, and you should be good. Short Hop FAirs and NAirs are part of Marth's approach tool (as CommanderPepper mention)s, not so much the side b. Side b is more for tech reading, like ftilt.
I'd recommend just learning what all of your options can do, especially the a attacks. Those ones are your go tos for answering every character in the game (and are part of the fundamentals xD). Like CommanderPepper, I hope this helps ^^
